Spinal fixation systems can be used in orthopedic surgery to align, stabilize, and/or fix a desired relationship between adjacent vertebral bodies. Such systems typically include a spinal fixation element, such as a relatively rigid fixation rod or plate, extending along an axis along which the vertebral bodies are to be positioned and coupled to adjacent vertebrae by attaching the element to various anchoring devices, such as hooks, bolts, wires, screws, etc. The spinal fixation element can have a predetermined contour that has been designed according to the properties of the target implantation site and, once installed, the spinal fixation element holds the vertebrae in a desired spatial relationship, either until desired healing or spinal fusion has occurred, or for some longer period of time.
Spinal fixation elements can be anchored to specific portions of the vertebra. Since each vertebra varies in shape and size, a variety of anchoring devices have been developed to facilitate engagement of a particular portion of the bone. Pedicle screw assemblies, for example, typically have a shape and size configured to engage pedicle bone, which is the strongest part of the vertebrae. Such screws typically include a threaded shank configured to be threaded into a vertebra, and a head portion having a spinal fixation element receiving element, which, in spinal rod applications, is typically in the form of a U-shaped slit formed in the head for receiving the rod. A closure mechanism such as a set-screw, plug, cap, etc. can be used to lock the rod into the receiving element of the pedicle screw. In conventional use, the shank portion of each screw is threaded into a vertebra, and once properly positioned, a fixation rod is seated through the receiving element of each screw, and the rod can be locked in place by tightening the closure mechanism to securely interconnect each screw and the fixation rod. Other anchoring devices include hooks and other types of bone screws
Placement of pedicle screws in a percutaneous fashion has become desirable in minimally invasive approaches to the spine. This technique generally relies heavily on a surgeon's clear understanding of a patient's local anatomy, as well as on accurate radiographic guidance technology. Generally, placement is done using a large bore needle or a cannulated drill to start an initial hole for screw placement. Pedicle screws are typically threaded in alignment with the pedicle axis and inserted along a trajectory that is determined prior to insertion of the screws. Misalignment of the pedicle screws during insertion can cause the screw body or its threads to break through the vertebral cortex and be in danger of striking surrounding nerve roots. One or more undesirable symptoms can easily arise when the screws make contact with nerves after breaking outside the pedicle cortex, such as dropped foot, neurological lesions, sensory deficits, and pain.
The placement of pedicle screws and other surgical implants for the spine and/or for other patient anatomies requires a high degree of accuracy and precision to ensure a proper trajectory for the implant. Each instrument used in the process is typically intended to be inserted along a same trajectory to ensure proper implant placement. Conventional surgical procedures for inserting pedicle screws involve recognizing landmarks along the spinal column for purposes of locating optimal screw hole entry points, approximating screw hole trajectories, and estimating proper screw hole depth. Generally, large amounts of fluoroscopy are required to determine a proper pedicle screw trajectory and to monitor the advancement of a pedicle screws through the vertebra. However, such techniques require prolonged radiation exposure to a patient and a surgeon, which risks undesirable effects of radiation exposure.
More technologically advanced systems such as the StealthStation® Treatment Guidance System, the Fluor® Nav® Virtual Fluoroscopy System (both available from Medtronic, Inc. of Minneapolis, Minn.), and related systems, seek to overcome the need for surgeons to approximate landmarks, angles, and trajectories, by assisting the surgeons in determining proper tap hole starting points, trajectories, and depths. However, these systems are extremely expensive, require significant training, are cumbersome in operation, are difficult to maintain, and are not cost effective for many hospitals and other surgical centers.
Accordingly, there remains a need for improved methods, systems, and devices for guiding surgical instruments.